A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells (SOFCs) use a hard, ceramic compound metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in solid oxide fuel cells, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2 gas, is oxidized with the oxygen ions to form water at the anode. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
Typically a SOFC stack consists of multiple cells connected in series configuration. The single cells consist of 3-5 layers, including a cathode, a cathode functional layer, an electrolyte, an anode functional layer, and an anode. The single cells are then assembled between metal interconnect plates and formed into a stack. One issue relates to the curvature of typical anode supported cells that are used in stacks. Even with optimized processing, a curvature of up to 150 microns is seen even for 5 cm×5 cm cells. A cell with curvature is shown in FIG. 1.
The curvature requires the application of a load to flatten the cells and achieve good contact with the metal interconnect plate (shown in FIG. 2). A good contact may provide low ohmic resistance and provide a good seal between the cell and the glass layer that is applied between the cell and interconnect. However, the large curvature leads to the need to have a high load (shown in FIG. 3) of up to 500 kg for 20 cm×20 cm cells that makes practical fabrication of small stacks very difficult. In addition, there are issues during thermal cycling due to the inherent tendency for the cells to curl during cooling and relax to a flatter profile on cooling. The dilation behavior leads to fatigue at the cell-glass seal interface during cycling and can lead to seal cracks. Cracks at the seal can then lead to gas leaks, local heating and progressive stack failure and power degradation.
The problem is even greater for large area cells which require larger loads to flatten the cell and also can lead to more seal failure issues from increased expansion issues at the seal-cell interface. Therefore, a need exists to minimize or eliminate the above mentioned problems.